This invention relates in general to a method for enhancing an image that is composed of two or more regions with different gray level concentrations, and more particularly to a method for visual enhancement of anatomical details in digital portal images that possess multiple X-ray exposure fields.
The American Cancer Society estimated that more than 1.2 million new cases of invasive cancer would be diagnosed in the United States in the year of 2000. Moreover, the estimate shows a doubling of cancer cases every 15 years. In the radiation treatment of cancer patients, portal imaging is an important tool for the optimization of treatment. Currently, digital imaging methods, e.g., computed radiography (CR), are being used to acquire digital portal images for radiation therapy. In this case, digital image processing methods can be used to optimally display the resultant portal images.
Portal images are used to evaluate the position of the radiation beam and the placement of x-ray radiation shielding blocks with respect to the patient""s anatomy. When portal images are taken before the treatment, they give the radiation oncologists the opportunity of correcting for minor patient positioning errors. When portal images are taken during treatment, they provide a means of monitoring patient movement. A portal image may be acquired by two (or more) radiation exposures.
A first exposure is taken only with the collimator mounted. Then the shielding blocks are mounted in front of the collimator before the second (or the following) exposure is taken. Interior of the shielding blocks is the treatment field, which is called the radiation field. The collimation field represents the interior of the collimator where the useful patient anatomic locations are recorded. There is no treatment information exterior of the collimation field. The image portions inside and outside the radiation field have different gray level concentrations because of different radiation doses that are applied to inside and outside the radiation field. Therefore, the edges of the radiation field appear relatively strong indicating the placement of the shielding blocks. However, because of the high energy of the radiation, digital portal images suffer from systematic low contrast.
It is desirable to increase the contrast of a patient""s anatomy inside and outside the radiation field. It is further desirable to reduce the differences of the gray level appearances inside and outside the radiation field such that the image details both inside and outside the radiation field are displayed using the full dynamic range of the output medium such as film or a high resolution display monitor. Moreover, the edges of the radiation filed should be preserved and even highlighted. Therefore, the processed portal images can provide radiation oncologists with the opportunity to check the treatment setup accurately and reduce localization errors.
Several methods of enhancing portal images were developed by Pizer""s group and Shalev and his co-workers (see S. M. Pizer, E. P. Amburn, J. D. Austin, et al. xe2x80x9cAdaptive histogram equalization and its variationsxe2x80x9d, Computer Vision, Graphic, and Image Processing, 39, pp. 355-368, 1987; G. W. Sherous, J. Rosenman, H. McMurry et al. xe2x80x9cAutomatic digital contrast enhancement of radiotherapy filmsxe2x80x9d, Int J Radiation Oncology Biol Phys, Vol. 13, pp. 801-806, 1987; J. Rosenman, C. A. Roe, R. Cromartie et al. xe2x80x9cPortal film enhancement: technique and clinical utilityxe2x80x9d, Int J Radiation Oncology Biol Phys, Vol. 25, pp. 333-338, 1993; K. W. Leszczynski, S. Shalev, and N. S. Cosby, xe2x80x9cThe enhancement of radiotherapy verification images by an automated edge detection techniquexe2x80x9d, Medical Physics, Vol. 19, no. 3, pp. 611-612, 1992). Their methods are based on histogram equalization and unsharp masking techniques. A significant drawback of using histogram equalization is the loss of definition on the edges of the radiation field. Unsharp masking is disadvantageous because it degrades edge information by causing edge banding effect around the edges of the radiation field. Shalev et al. developed a method to delineate the radiation field while still using the histogram equalization method. Yet, the image inside and outside the radiation field still contained different gray level concentrations. This limited the image contrast enhancement when displayed on an output medium.
Another method of enhancing the contrast of a portal image is described by Cheng (see U.S. Pat. No. 6,094,152, xe2x80x9cAlgorithm for A/D window control for electronic portal image acquisition in a radiotherapy systemxe2x80x9d, Jul. 25, 2000, F. T. Cheng). This method is designed to increase the signal-to-noise ratio of the portal images during the image acquisition stage. This is an important operation. But it is different from the digital image processing technique of the present invention, which enhance image contrast after the image acquisition.
According to the present invention, there is provided a solution to the problems and fulfillment of the needs discussed above.
According to a feature of the present invention, there is provided a method for enhancing the image contrast of digital portal images for presentation on an output medium comprising: providing an input digital portal image having radiation and collimation fields; locating and labeling said radiation and collimation fields to produce labeled radiation and labeled collimation field images; designing a tone scale curve to display the image inside the radiation field using the full dynamic range of the output medium; applying said tone scale curve to the input digital portal image to produce a tone-scaled radiation field image; designing a tone scale curve to display the image outside the radiation field using the full dynamic range of said output medium; applying said tone scale curve to the input digital portal image to produce a tone-scaled collimation field image; enhancing the image contrast of said tone-scaled radiation field image and said tone-scaled collimation field image; combining said enhanced images using the labeled radiation and collimation field images; and black-surrounding the collimation field to produce a contrast enhancement output image which can be presented on said output medium.
The invention provides a general method of enhancing the image contrast in a digital portal image without adding the edge-banding artifacts around high-contrast edges. The invention has the following advantages.
1. The image details of the patient anatomy both inside and outside radiation field are enhanced.
2. The image details of the patient anatomy both inside and outside the radiation field are displayed using the full dynamic range of the output medium.
3. The edges of the radiation field are preserved without degradation in the output digital portal image.